Commutating brush having improved resistance and riding characteristics

ABSTRACT

The invention comprises an electrographitic commutating brush having a low electrical resistance characteristic in a direction substantially perpendicular to a working face of the brush, and a high electrical resistance characteristic in the direction of brush movement relative to a commutator. To produce such characteristic resistances, the body of the brush is provided with voids or cavities controlled in size and shape, and oriented to have elongated dimensions in planes perpendicular to the working face of the brush, and in planes perpendicular to the direction of relative brush rotation with the commutator.

United States Patent Inventor Lawrence E. Moberly Pittsburgh, Pa.

Sept. 22, 1969 June 29, 1971 Westinghouse Electric Corporation Pittsburgh, Pa.

Continuation of application Ser. No. 682,391, Nov. 13, I967, now abandoned.

Appl. No: Filed Patented Assignee COMMUTATING BRUSH HAVING IMPROVED RESISTANCE AND RIDING CHARACTERISTICS 4 Claims, 3 Drawing Figs.

u.s.'c| 310/248 Int. Cl. H02k 13/00 Field of Search 310/219,

[561 References Cited UNITED STATES PATENTS 2,125,027 7/1938 Kasperowskl 1 310/248 3,114,062 12/1963 Fay 310/253 FOREIGN PATENTS 138,450 1/1903 I Germany 310/248 Primary Examiner-Milton O. Hirshifield Assistant ExaminerR. Skudy Attorneys A. .l. Stratton. F. P. Lyle, and E. Strickland ABSTRACT: The invention comprises an electrographitic commutating brush having a low electrical resistance characteristic in a direction substantially perpendicular to a working face of the brush, and a high electrical resistance characteristic in the direction of brush movement relative to a commutator. To produce such characteristic resistances, the body of the brush is provided with voids or cavities controlled in size and shape, and oriented to have elongated dimensions in planes perpendicular to the working face of the brush, and in planes perpendicular to the direction of relative brush rotation with the commutator.

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z W F m ence VENTO Lowr E. Mobe ly K 195 ATTO NEY COMMUTATING BRUSH HAVING IMPROVED RESISTANCE AND RIDING CHARACTERISTICS This application is a continuation of my copending application Ser. No. 682,39l,filed Nov. 13, i967, now abandoned.

BACKGROUND OF THE INVENTION The present invention relates generally to brushes for use in dynamoelectric machines, and particularly to an economical brush structure designed to substantially reduce circulating currents in armature windings shorted by the brush while simultaneously efficiently handling high load currents.

In the design of electrical machines using commutating means, limitations in regard to speed, power rating and size are encountered due to electrical and thermal characteristics of available carbon brushes. For example, the commutating (current reversing) range of brushes is generally reduced as electrical load and rotating speed of machines are increased so that brushes with a wide range of commutating efi'rciency are in demand.

A major factor in commutation efficiency is the capability of the brush to minimize circulating current in the associated armature windings undergoing commutation while simultaneously transferring high load currents without excessive heatmg. v

Heretofore, progress was made with the use of brushes made of electrographitic carbon materials exhibiting a relatively high electrical resistance, as compared to the available graphite brush, thereby tending to reduce the undesirable circulating currents. The electrographitic brush was further found to have improved life and friction loss characteristics so that electrographitic carbons have been highly favored for use in making commutating brushes.

A limiting factor, however, in the use of electrographitic carbons, is that the electrical resistance characteristic of such materials is highly isotropic so that when gains in commutating efficiency are sought through adjustment increases in the resistance of the carbons, a loss in electrical load carrying capacity of the brush results, and a limiting compromise is reached by brush designers.

What is needed, therefore, is an electrographitic carbon brush which will exhibit a low resistance in the direction perpendicular to the working face of the brush, thereby allowing it to conduct high load currents efficiently (i.e., without high heat losses), and a high resistance in the direction of brush movement relative to its associated commutator thereby limiting circulating currents to provide greater commutating effectiveness. This combination of resistance properties has heretofore been unobtainable with electrographitic brushes which otherwise have very desirable characteristics as explained above.

BRIEF SUMMARY OF THE INVENTION The present invention comprises an electrographitic brush structure having the above combination of high and low resistance properties as well as a more elastic and therefore better riding property, such properties being provided in the brush in a simple and economical manner. Briefly, the resistance and better riding properties are produced in the brush by forming elongated voids or cavities therein, the elongated dimension of the cavities extending in a direction perpendicular to the commutator engaging face of the brush and perpendicular to the direction of relative brush movement with the commutator.

The voids or cavities may be easily provided in the brush by the method shown and described in US. Pat. No. 2,356,076 issued to the present inventor and assigned to present assignee. The method includes disposing particles ofa solid substance in a moldable carbon flour, the solid substance being selected for its property of changing into a gas at predetermined temperatures. The flour (comprising a mixture of carbon powders and organic binder) is formed into plates by molding under various combinations of pressure and temperature, and during a heat treatment for carbonizing the organic binder, the solid substance volatilizes leaving cavities conforming to the shape and size of the original particles.

In the present invention, flakes or sheets of thin organic film materials are disposed in the carbon flour, the films selected on the basis of their thermally stable characteristics for carbon plate and bond setting processes, and for their thermally unstable characteristics under higher temperature conditions, at which time, the flakes or sheets decompose and volatilize leaving zero or inconsequential amounts of residue with a pore space conforming to the size, shape and orientation of the original flakes or sheets.

THE DRAWING The objects and advantages of the invention will be apparent from reading the following detailed description taken in connection with the accompanying drawing in which:

FIG. 1 is a longitudinal sectional view of a brush made in accordance with the invention;

FIG. 2 is a longitudinal sectional view of another embodiment of the invention; and

FIG. 3 is a perspective view of a third embodiment of the invention.

PREFERRED EMBODIMENTS Specifically, there is shown in FIG. 1, a sectional view of a commutating brush body 10 having a working face 12 engaging a commutator or slip ring 14 only representively shown.

The brush body 10 is provided with a controlled multiplicity of thin cavities 16, the cavities being represented by short lines relatively uniformly distributed throughout the brush body. The cavities are further formed to extend in planes parallel to each other and parallel to the width and length dimensions of the brush body; the cavities further lie in planes perpendicular to the direction of commutator rotation, as indicated by the arrow, and to the plane of the working face 12 of the brush.

The thin, oriented cavities 16, as diagrammatically shown in FIG. I, substantially limit the amount of current carrying mass of the brush in a transverse direction, i.e., in a direction perpendicular to the commutator l4 rotation. Thus, in the transverse direction, a high electrical resistance is provided which limits current flow so that when the brush 10 is bridging (shorting) two adjacent commutator segments and their associate armature coils (not shown), current flow in the transverse direction is negligible. Undesirable armature circulating currents are thus essentially eliminated, or greatly reduced, thereby providing greater commutating capability.

The cavities 16 do not limit current flow in the brush 10 in a longitudinal direction since the current carrying mass of the brush is substantially solid in such direction, i.e., in a direction perpendicular to the working face 12 of the brush and the actual cross-sectional area of carbon is not materially reduced by the item cavities. Thus, the brush 10 can effectively handle large load currents with minimum energy losses.

The thin cavities 16 are formed in the brush body 10 by blending a controlled quantity of flakes of a solid material (not shown) in a quantity of moldable carbon flour comprising a mixture of carbon powders and organic binder, the solid material having a characteristic of changing into a gas at a predetermined temperature. Specific solid materials having such a characteristic include thin organic films such as cellulose triacetate, cellophane, polyethylene, polypropylene and others. The flakes of film material may be punched, cut, chopped or otherwise sectioned to form the desired size and shape.

The flakes are disposed in the moldable carbon flour to form a random dispersion of flakes therein. The resulting mixture of flour and flakes is then placed in a plate mold (not shown) and mechanically vibrated to properly orient the flakes in the desired manner, namely, an orientation in which the elongated dimension of the flakes will be in planes perpendicular to the direction of relative brush rotation and perpendicular to the commutator engaging face 12 of the'brush body to be molded.

The carbon flour mixture is next formed into solid plates in plate molds by molding under various combinations of pressure and temperature to give desired strengths, densities and other properties. The molded plates are then fired under exacting conditions to carbonize the binder and then heated to graphitizing temperatures to produce the desired electrographitic qualities. The above described heat treatments may take place in an electric furnace (hence the name electrographitic) such as shown and described in the above mentioned patent, or in other suitable heat treating means.

During the heat treatment for carbonizing the organic binder, the flakes of organic films volatilize leaving voids or cavities 16 conforming to the size, shape and orientation of the original flakes. These predetermined cavity structures are preserved throughout subsequent processing of the molded plate structure since the basic form of the plate becomes rigid with the carbonization of the binder.

FIG. 2 shows another embodiment of the invention in which a brush body 20 is provided with sheetlike voids 18. The voids are produced by utilizing perforated sheets of the above mentioned thin organic films in the molding flour, and ultimately evaporating the films by the process described above.

The brushes 20 are made from molded plates of carbon flour. The plates are made by first disposing alternate layers of the thin organic film and the molding flour in the above mentioned plate mold for pressing into a multiple sandwich plate. The carbon flour may be added as a loose flour or as thin preformed sheets. Bonding between the layer or sheets or carbon flour is obtained through the holes in the perforated films.

Control of the electrical resistance properties of the brush 20 is obtained through selection of perforation geometry, concentration of the perforations, thickness of the film and the spacing between the sheets of film. If desired, only the working end of the brush need be made to contain the voids 18. To provide extra strength-at the other (nonworking) end of the brush for attaching brush shunts or hardware, the pore producing sheets of organic film can be confined to the vicinity of the working section of the brush.

F IG. 3 shows an embodiment of the invention in which solid nonperforated sheets of film material are used to provide thin slot openings 28 in a brush body 30. By properly locating the sheets in the molding mixture prior to plate forming, as described above, the brush 30 is produced having a solid end section 32 and a working end section 34 in which the slot openings 28, left by the evaporating film layers, extend inwardly from the commutating end 12 of the brush and across the width thereof.

Like the cavities 16 and 18 in the brushes [0 and 20 respectively, the slot openings 28 produce a high electrical resistance in the brush 30 in a thickness direction relative to that in length and width directions.

Electrographic brush materials, as explained above, can be made highly conductive so that they have an excellent load carrying capacity but a limited commutating capacity. The above described structure provides an electrographic brush with an excellent commutating capacity while simultaneously preserving its high current carrying capacity. Further, the commutating range of presently available, adequate-commutating electrographitic brushes can be increased even further by the invention. And, as mentioned earlier, the brush structure of the invention provides an increased resiliency which improves riding quality, i.e., the ability of the brush to maintain electrical contact with the rotating commutator or slip ring.

Though the invention has been described with a certain degree of particularity, it is to be understood that the disclosure has been made by way of example only and that changes may be made therein without departing from the scope and spirit of the invention.

What 1 claim is: l. A carbon brush for engaging a commutator, said brush comprising a brush body of electrographitic material having a commutator engaging face at one end thereof and having a multiplicity of voids therein, said voids being distributed substantially uniformly across the entire width of the brush body and throughout the brush body and being oriented in a plurali- 'ty of parallel planes extending perpendicular to said commutator engaging face, said voids being elongated in the direction of said planes and thin in the transverse direction as compared to the thickness of material between the voids.

2. A carbon brush as defined in claim 1 in which said voids .are spaces resulting from vaporization of spaced parallel sheets of volatilizable material disposed in the brush body.

3. A carbon brush as defined in claim 1 in which the voids are continuous throughout each of said planes but with the brush material extending through the voids in spaced locations.

4. A carbon brush as defined in claim 1 in which the voids are continuous in each of said planes and extend to the commutator engaging face to form a series of thin slots extending at least part way through the brush. 

1. A carbon brush for engaging a commutator, said brush comprising a brush body of electrographitic material having a commutator engaging face at one end thereof and having a multiplicity of voids therein, said voids being distributed substantially uniformly across the entire width of the brush body and throughout the brush body and being oriented in a plurality of parallel planes extending perpendicular to said commutator engaging face, said voids being elongated in the direction of said planes and thin in the transverse direction as compared to the thickness of material between the voids.
 2. A carbon brush as defined in claim 1 in which said voids are spaces resulting from vaporization of spaced parallel sheets of volatilizable material disposed in the brush body.
 3. A carbon brush as defined in claim 1 in which the voids are continuous throughout each of said planes but with the brush material extending through the voids in spaced locations.
 4. A carbon brush as defined in claim 1 in which the voids are continuous in each of said planes and extend to the commutator engaging face to form a series of thin slots extending at least part way through the brush. 